5.
P2P Computing*
n P2P computing is the sharing of computer resources and
services by direct exchange between systems.
n These resources and services include the exchange of
information, processing cycles, cache storage, and disk
storage for files.
n P2P computing takes advantage of existing computing
power, computer storage and networking connectivity,
allowing users to leverage their collective power to the
‘benefit’ of all.
* From http://www-sop.inria.fr/mistral/personnel/Robin.Groenevelt/
Publications/Peer-to-Peer_Introduction_Feb.ppt

13.
Gnutella
n Share any type of files
(not just music)
n Decentralized search
unlike Napster
n You ask your neighbours
for files of interest
n Neighbours ask their
neighbours, and so on
n TTL field quenches
messages after a
number of hops
n Users with matching files
reply to you
* Figure from http://computer.howstuffworks.com/file-sharing.htm

14.
Gnutella
n Decentralized
n No single point of failure
n Not as susceptible to denial of service
n Cannot ensure correct results
n Flooding queries
n Search is now distributed but still not scalable

15.
Kazaa (Fasttrack network)
n Hybrid of centralized Napster and decentralized Gnutella
n Super-peers act as local search hubs
n Each super-peer is similar to a Napster server for a small portion of
the network
n Super-peers are automatically chosen by the system based on their
capacities (storage, bandwidth, etc.) and availability (connection
time)
n Users upload their list of files to a super-peer
n Super-peers periodically exchange file lists
n You send queries to a super-peer for files of interest

17.
Anonymity
n Napster, Gnutella, Kazaa don’t provide
anonymity
n Users know who they are downloading from
n Others know who sent a query
n Freenet
n Designed to provide anonymity among other
features

18.
Freenet
n Data flows in reverse path of query
n Impossible to know if a user is initiating or forwarding a query
n Impossible to know if a user is consuming or forwarding data
n “Smart” queries
n Requests get
routed to
correct peer
by
incremental
discovery

23.
DHT Desirable Properties
n Keys mapped evenly to all nodes in the
network
n Each node maintains information about only
a few other nodes
n Messages can be routed to a node
efficiently
n Node arrival/departures only affect a few
nodes

26.
Chord Identifier Circle
n Nodes organized in
an identifier circle
based on node
identifiers
n Keys assigned to
their successor node
in the identifier circle
n Hash function
ensures even
distribution of nodes
and keys on the
circle

29.
Chord Properties
n In a system with N nodes and K keys, with
high probability…
n each node receives at most K/N keys
n each node maintains info. about O(logN) other
nodes
n lookups resolved with O(logN) hops
n No delivery guarantees
n No consistency among replicas
n Hops have poor network locality

35.
Conclusions
n P2P connects devices at the edge of the Internet
n Popular in “industry”
n Napster, Kazaa, etc. allow users to share data
n Legal issues still to be resolved
n Exciting research in academia
n DHTs (Chord, Pastry, etc.)
n Improve properties/performance of overlays
n Applications other than file sharing are being developed